This invention relates to a thermal fixing device in an image forming apparatus.
A conventional fixing device in an image forming apparatus utilizing an electrophotographic process is a thermal fixing device which is made up of a pair of rollers which are rotated while being pushed against each other. In order to improve the thermal efficiency and the image fixing characteristic of the device, at least one of the rollers has a heat generating unit. Temperature detecting means such as a thermistor element is held in contact with the surface of the roller having the heat generating unit (hereinafter referred to as a "heat roller"). The temperature detecting means cooperates with a temperature controlling power source to set the surface temperature of the heat roller to an image fixing temperature. A transfer sheet on which a toner image has been transferred is passed through the rollers thus heated, so that the toner image is fixed on the transfer sheet by heat and pressure.
The heat roller is generally formed as follows: A sheathed heater or halogen lamp heater is fixedly inserted in a metal roller. However, the heat roller thus formed is disadvantageous in the following points: A period of time required for increasing the temperature of the roller to an image fixing temperature, about 130.degree., is relatively long, e.g., more than one minute. In addition, the heating element is high in temperature 250.degree. to 300.degree. C. or higher. Accordingly, the surface temperate of the heat roller is variable in a wide range, so that an unwanted offset phenomenon is liable to occur. Moreover, for the same reason, the power consumption is large, e.g. more than 800 watts.
In order to eliminate the above-described difficulties, the following thermal fixing system has been proposed in the art (cf. Japanese Patent Application Publication No. 17061/1978, and U.S. Pat. No. 3,811,828) stationary heat generating unit is brought into close contact with a transfer sheet, on which a toner image is formed so as to be fixed, through a thin endless film of heat-resisting resin which is moved in synchronization with the transfer sheet. A concrete example of the system has been disclosed in a Japanese Electrophotographic Society, June 1990. More specifically, the concrete example of the system has been disclosed as a SURF system by Canon (Co., Ltd.) In the system, when the image fixing speed is 6 cpm (copies per minute)/A4, and a period of time required for obtaining an image fixing temperature (hereinafter referred to as "a temperature rise time", when applicable) is five seconds or less, the stationary heat generating unit is 180.degree. to 190.degree. C., and the necessary heating electric power is 400 to 450 watts. These numerical values indicate that the SURF system is novel in performance.
Listed below, substantially in the order of introduction in the art, are conventional methods concerning the SURF system:
(1) First method: A stationary heat generating element at a predetermined temperature is pushed against a fixing sheet through a thin endless belt of resin which is moved at the same speed as the fixing sheet, so as to subject the toner on the fixing sheet to fixing. The method aims to reduce the period of time required for starting the fixing operation, and to decrease the power consumption. (U.S. Pat. 3,811,828)
(2) Second method: A thermally conductive endless belt is laid over a heating roller and a guide roller. A fixing sheet is loaded on the belt on the side of the heating roller. The fixing sheet thus loaded is conveyed while being pushed against the belt, and is then unloaded from the belt on the side of the guide roller. During this operation, the toner on the fixing sheet which has not been fixed yet is molten at the vicinity of the heating roller, and it is cooled while the fixing sheet is being conveyed to the vicinity of the guide roller. The method aims to reduce the amount of offset. (U.S. Pat. No. 3,578,797)
(3) Third method: In the second method, the thermally conductive endless belt is made of "Teflon" or metal. (Japanese Patent Application (OPI) No. 70633/1974 (the term "OPI" as used herein means an "unexamined published application")
(4) Fourth method: An endless belt of metal is laid over two rollers. The endless belt is heated with a lamp, or by the direct application of electric current or by electromagnetic induction. From outside of the endless belt, a fixing sheet with toner is inserted between one of the two rollers and a pressure roller abutted against it, so as to fix the toner on the fixing sheet. (Japanese Utility Patent Application (OPI) No's 116961/1982, 190659/1983 and 68665/1988, and Japanese Patent Application (OPI) No. 144084/1989)
(5) Fifth method: A stationary heat generating element which performs a heating operation and a cooling operation repeatedly in a pulse mode is pushed against a fixing sheet through an endless belt of resin which is moved at the same speed as the fixing sheet, to melt and solidify the toner on the fixing sheet which has not yet been fixed. (Japanese Patent Application (OPI) No's 313182/1988, 263677/1989, and 263680/1989)
(6) Sixth method: A stationary heating element which is formed integral with a PTC heat generating element and is maintained at a predetermined temperature is pushed against a fixing sheet, on which toner is provided, through an endless belt of resin which is moved at the same speed as the fixing sheet, to fix the toner on the fixing sheet. (Japanese Patent Application (OPI) No's 263679/1989 and 158782/1990)
Of the above-described methods, the first method is a fundamental patent on a belt method. This idea has been put in practical use as the SURF system by Canon (Co., Ltd.) for the first time.
The second method can be considered as a fundamental patent on a heating and cooling method in which an amount of offset can be decreased most readily. However, it has not been practiced yet, apparently because it takes a relatively long period of time to cool the toner and the heated fixing sheet's surface down to a temperature at which the toner viscosity is sufficiently large, with results that the fixing unit is unavoidably bulky and the fixing speed is low.
The third method is similar to the second method. In this method, the belt may be a mirror-finished thin endless belt of stainless steel as well as the endless belt of "Teflon". The method, however, has not been practiced yet.
The fourth method is one of conventional methods of heating a metal belt; however, it is difficult to practice. The object of the method is not clear and it has not apparently been practiced yet.
In the fifth method, the toner on the fixing sheet which is pushed through the resin belt by the stationary heat generating element, is heated by application of pulse current for a short period of time so that it is molten. Thereafter, the toner thus molten is cooled for a period of time several times longer than the period of time required for application of the pulse current so that the molten toner is increased in viscosity. The fixing sheet is conveyed at a predetermined rate; that is, it is conveyed a distance corresponding to the width of the stationary heat generating element each cycle in which the application of the pulse current and the cooling of the toner are carried out. In practice, it is necessary that the distance of movement of the fixing sheet is smaller than the width of the stationary heat generating element. It is apparent that the endless belt of resin should be about 30 .mu.m or more, although its concrete example (a fixing unit of SURF system) has not been referred to. Hence, in order to heat and cool the toner through the endless belt of resin which is thick as described above and accordingly low in thermal conduction, it is necessary to significantly decrease the fixing speed. Thus, the method is not practical. This is the reason why Canon (Co., Ltd.) has employed the first method instead of the fifth method as the SURF system.
The sixth method relates to the heating source in the stationary heat generating element in the first method. The invention relates to a method of providing a heating zone and a cooling zone; however, it can be considered that the sixth method is similar to the second method. The sixth method has not been practiced yet.
In the first method, as proven by the SURF system, the waiting time, which elapses until the start of the fixing operation, can be greatly reduced, and the power consumption can be greatly decreased. This excellent invention was put into practical use twenty years ago, because it was then that it became possible to manufacture a thin endless belt using polyimide high rigidity and high in heat resistance. In order to prevent the occurrence of offset, it is essential to form a film of release agent such as PTFE on the surface of the polyimide belt. The belt can be used until the film thus formed peels off. More specifically, the belt can be used for printing about 50,000 sheets of paper of A4 size. This is less than half (1/2) of the average service life of the conventional heat roller type fixing unit.
On the other hand, when an endless belt is driven, in general, it moves from side to side. In order to control such side to side movement the belt, its thickness must be reduced, although a reduction of the thickness of the belt is limited to some extent. In the case of the SURF system, the thickness of the belt is set to about 30 .mu.m including the PTFE layer. This thickness impedes the transmission of heat from the stationary heat generating element, and accordingly it is necessary to set the temperature of the stationary heat generating element to a value much higher than the toner fixing temperature.
The capacity of heating through the PTFE/polyimide double layer belt of this thickness depends on the heat resistance of polyimide material, and it is estimated that a fixing capacity of 6 to 10 cpm (copies per minute)/A4 is the limitations in the SURF system. The low thermal conduction of the double layer belt makes it impossible to put the second method to practical use, and, in practicing the fifth method, causes the fixing speed to be set to a value which is too slow to be practical.
As was described above, the SURF system is a novel system which is advantageous in that the temperature-rise time is short, and the power consumption is small. The system has many other excellent advantages. However, the SURF system still suffers from problems in that the thermal fixing device has a relatively short service life because of the short service life of the thin endless polyimide film (50,000 sheets of size A4 paper--the application of the SURF system is limited to a low speed thermal fixing device), yet the thermal fixing device is intricate in construction and accordingly high in manufacturing cost. In addition, the temperature of the stationary heat generating unit is much higher than required for the fixing operation. Thus, it is necessary to decrease the temperature of the stationary heat generating unit and to further decrease the electric power required for heating it.